This invention relates to optical fibre cable manufacture and more particularly to a method of manufacturing an optical fibre cable comprising at least one optical fibre unit disposed within an extruded sheath with water absorbent powder being provided between the optical fibre unit or units and the sheath.
Such cables are known where the water absorbent powder is provided to prevent longitudinal water migration within the sheath. In the manufacture of these cables the water absorbent powder is provided within the sheath by being metered directly into the extruder cross-head and/or transported on the optical fibre units by electrostatic attraction thereto. In order to improve the degree of water migration prevention, for example to prevent transverse water migration as well as longitudinal water migration, there is a need to provide more water absorbing powder within the sheath than generally achievable by these known techniques.
The present invention provides a method of manufacturing an optical fibre cable comprising at least one optical fibre unit disposed within an extruded sheath with water absorbent powder being provided between the optical fibre unit or units and the sheath, in which method said at least one fibre unit passes through an extruder cross-head extruding said sheath together with a tape transporting the water absorbent powder, the tape being formed upstream of said extruder crosshead into a channel into which said water absorbent powder is delivered.
The invention also includes an optical fibre cable when made by a method as defined in the last preceding paragraph.
The tape may be adhered to the inside surface of the sheath and in this case preferably the tape is adhered to the inside surface of said sheath by fusion thereto.
To this end, preferably the material of the tape adjacent the inside surface of the sheath has a fusion temperature below that of the temperature of the extrusion material in the extrusion cross-head.
The tape may be a laminate of at least two layers, the material of the layer adjacent the inner surface of the sheath having a fusion temperature below that of the temperature of the extrusion material in the extrusion cross-head and the material of the, or at least one, other layer having a fusion temperature above that of the temperature of the extrusion material in the extrusion cross-head.
In a presently preferred embodiment, the tape is a laminate of two layers, the material of the layer adjacent the inner surface of the sheath being a polyethylene copolymer. In this embodiment the material of the other layer is a polyester. Furthermore, in this embodiment the extrusion material is a medium density polyethylene.
Preferably, the channel is arcuate in transverse cross-section with substantially the same radius of curvature as the inside surface of the sheath.
In the above-mentioned embodiment, the channel is substantially semi-circular in transverse cross-section.
In carrying out the method, the rate of delivery of water absorbent powder into the channel is preferably controlled depending on the space between the sheath and the fibre unit or units in the cable design and the rate of extrusion of the sheath. It is to be noted that said at least one fibre unit of the cable may comprise a plurality of optical fibres disposed within a tube. It is to be understood, however, that the fibre unit may take other forms. For example, said at least one fibre unit may comprise a single optical fibre provided with a protective coating, or a ribbon of optical fibres, which may be disposed within a tube.
The invention also includes an optical fibre cable comprising at least one optical fibre unit disposed within an extruded sheath with water absorbent powder provided between the optical fibre unit or units and the sheath, wherein a longitudinally extending tape is disposed adjacent the inside surface of the sheath over only a portion of the peripheral extent thereof.